The present invention relates to an intense soft X-ray source using a plasma microchannel obtained by the photoionization of a gas.
The plasmas to which the invention relates are dense, hot plasmas. Their electron density exceeds approximately 10.sup.18 cm.sup.-3 and their electron temperature is in the range between a few hundred electron volts to a few kiloelectron volts.
Such plasmas can constitute intense soft X-radiation sources which, compared with other X-ray sources have numerous advantages, such as:
their low cost, PA1 their overall dimensions are sufficiently reduced to enable them to be positioned at the point of use of the X-radiation, PA1 easy to use and maintain, PA1 high energy efficiency.
These advantages make such sources suitable for microlithography, whilst also being usable in fast X-ray microscopy.
These sources include those involving an intense electrical discharge through a plasma microchannel, which is heated by the Joule effect and becomes the seat of an intense emission of soft X-rays.
However, despite their interest, these sources are confronted by the following difficulty. A dense, hot plasma has a resistivity which, in the absence of turbulence or instabilities, can be described by the Spitzer formula, which is only dependent on the temperature and is in the form: EQU 10.sup.-3 /(kT).sup.3/2
in which k is the Boltzmann constant and T is the temperature.
The variation of the resistivity in 1/T.sup.3/2 makes it very difficult to heat a plasma by the Joule effect because, as the temperature rises, the resistivity falls and so does the Joule effect. However, this heating remains possible on using a plasma microchannel, i.e. a channel with a very small diameter. This procedure is presently used in controlled thermonuclear fusion.
In order to produce a very fine plasma channel, the use is presently made of ionization by the laser breakdown of a high pressure gas or by the photoionization of this gas under the influence of a small diameter laser beam. The thus obtained plasma line also has a diameter of a few hundred microns, but it would appear difficult to drop below this.
For example, devices of this type have been described in the paper entitled "Recent results on dense Z pinches" given by J. E. HAMMEL et al at the "Symposium on new trends in unconventional approaches to magnetic fusion", given at the Royal Institute of Technology, Stockholm, June 16 to 18, 1972.
An important difficulty encountered in research on controlled thermonuclear fusion is that of confining a hot, dense plasma for a sufficient time to satisfy the Lawson criterion. This difficulty no longer appears in the application to the formation of soft X-radiation when using pulsed sources, which can e.g. be employed in microlithography.